A new discovery made me think of what has been done before. In one case it was officially announced; in another case by urgent need and now a sneaky example?
a- South Africa: forerunner due to demand (15-07-1913 to 31-08-1913)
In this case , the current stock of colonial revenues was running dangerously low. This was especially for the values 3d; 6d and £1. The amount of several values was already available for issue on September 1st of 1913. Based on a communication between the Department providing the stamps and distributors, They wrote a letter on July 15th declaring the pre-issue of these values. The SA Philatelist of December 2017 has a marvellous article about this series written by Dr. Gerhard Kamffer. He states that the 1st day of pre-issue was 17th of July but a stamp showing this date is missing.